1. The courtship behaviour of Drosophila melanogaster and D. simulans has been studied. Melanogaster is a generally more active species than simulans and its courtship is more vigorous. 2. The sexual behaviour of both types of male can be readily classified into the same basic elements, orientation, wing display, licking and attempted copulation. 3. Simulans males have longer lag periods before courtship, longer bouts of simple orientation, fewer changes of element when courting and their chief wing display is 'scissoring'. Melanogaster's chief wing display is 'vibration'. 4. Experiments in which the sexual stimulus situation was changed show that with lower stimulation melanogaster males show an increased proportion of scissoring, while with increased stimulation simulans males show more vibration. Additional reasons are given for considering scissoring to represent a lower level of sexual excitation than vibration. 5. The differences between the males can be most simply ascribed to simulans having a slower rise of sexual excitation during courtship than melanogaster, not to any changes in the basic organization of their behaviour. This slowed rise may be related to higher thresholds of response in simulans. 6. The females of simulans are more responsive to visual aspects of their males' courtship, and less responsive to those stimuli perceived by their antennae, than are melanogaster females. 7. It is argued that these changes in courtship behaviour probably resulted from selection acting upon a common ancestral population so as to favour high activity in some environments and low activity in others. These changes will be most readily produced by mutations affecting reaction thresholds and the changes to courtship pattern will follow. Selection will then favour females responsive to the most easily available stimuli from the males' courtship. 8. It is suggested that similar situations occur in other sibling Drosophila species and that as a first step in behavioural divergence, mutations affecting thresholds will be particularly important.